This invention relates to novel silicate based drilling fluids to be used primarily in well drilling for oil and gas. In particular this invention concerns the formulation of a silicate based drilling fluid that provides greater lubrication during drilling and can be used as a high pressure and high temperature rheology agent.
The use of silicate as a drilling fluid component is well established. Silicate has been used since the 1930s as an effective means of stabilizing shale formations. Despite being an effective shale stabilizer, silicate never achieved early, widespread success, owing to certain advantages held by oil based drilling fluids. Oil based drilling fluids offer ease of use, are not prone to gellation or precipitation and provide good lubrication between the drill string and well bore, as measured by coefficients of friction in the range of 0.10 to 0.18 compared with a range of 0.18 to 0.22 for water-based fluids. Until recent environmental pressures there was little incentive to improve the performance deficiencies in silicate-containing, water-based drilling fluids.
An early example of the use of silicate in the water-based drilling fluid is afforded by U.S. Pat. No. 2,133,759 in which the shale inhibition characteristics of silicate are described.
Elsewhere in the prior art, there is discussed the use of alkyl ammonium in compounds in drilling fluids for enhancing performance. In particular, U.S. Pat. No. 4,637,883 discusses the reaction of quebracho tannin with alkyl quaternary ammonium salts in an oil-based drilling fluid. The function of the alkyl quaternary ammonium salt-quebracho complex is to act as a high-temperature, high-pressure fluid loss additive in an oil-based drilling fluid.
European Patent EP-A-0 445 677 (equivalent to U.S. Pat. No. 5,380,706) describes a quaternary ammonium compound used for shale inhibition in a water-based drilling fluid. European Patent EP-A-0 390 387 describes the use of tetra alkyl ammonium compounds in the formulation of a hydraulic fracturing fluid, where they prevent the adsorption of clay onto polysaccharides. British Patent Publication GB-A-949058 describes the use of an oil-based fracturing fluid in which surface active agents are employed, including tertiary amines, to disperse oil-insoluble solids.
The first aspect of this invention deals with improving the lubricating properties of silicate by the addition of a lubricant which remains effective in highly alkaline silicate media. Good lubrication is necessary in order to avoid slower drilling rates, differential sticking, higher torque, fatigue on the drill string and possible hole loss. These problems become more acute with increases in well hole length and/or the hole angle.
We have discovered that the addition of certain tetra alkyl ammonium compounds, in particular tetra (lower alkyl) ammonium hydroxide, to silicate or silicate drilling fluid mud can lower the co-efficient of friction between the drill string and well bore. As well as meeting lubricant requirements, tetra alkyl ammonium compounds are environmentally safe, compatible with other drilling additives and temperature and time stable.
The second aspect of this invention deals with imparting stable high pressure and high temperature (HPHT) rheology to a silicate drilling fluid. The rheology of a drilling fluid is a critical factor governing such drilling characteristics as removal of drill cuttings, stabilization of the rock formation, ease of pumping, control of fluid loss and drill string support. It is critical that a desirable rheology be maintained during the full range of temperatures encountered during the drilling procedure. Maintenance of rheology becomes increasingly difficult as higher temperatures are encountered downhole.
Although silicate itself is stable at HPHT, the viscosity increasing organic polymeric additives typically used to impart rheological properties and to control fluid loss are by themselves subject to breakdown at HPHT. We have discovered that the combination of the aforementioned tetra alkyl ammonium compounds with silicate and, additionally, a hectorite clay, results in a useful, HPHT-stable rheology for drilling. It is believed that other applications in which control of rheology is important, such as coatings and adhesives, might well benefit from the discovery of this synergic enhancement of properties, quite apart from the specific present application of rheology agents for drilling fluids.
Swelling clays such as bentonite are used extensively in the drilling fluid industry as rheology agents. As well as providing a useful rheology, swelling clays are resistant to HPHT. Swelling clays had not been used with silicate-based drilling fluids, however, because the addition of a small amount of silicate to a swelled clay often results in total loss of viscosity of the swelled clay. That loss of viscosity or rheological properties is due to the dispersion effect silicate has on clay platelets.
Swelled hectorite clay, on the other hand, apparently can retain partial viscosity after the addition of silicate, with the loss of viscosity in a swelled hectorite clay proportional to the amount of silicate added. We have discovered that inclusion of a tetra alkyl ammonium compound reduces the dispersion effect of silicate has on the swelled hectorite, thus allowing for a greater range of formulating hectorite with silicate in drilling fluids.
A further advantageous effect of the addition of tetra alkyl ammonium hydroxide to silicate-based drilling fluids containing hectorite clays is that the resulting modification to the rheology and the silicate/clay mud produces a drilling fluid more comparable in behavior, in low- to medium-pressure and temperature wells, to conventional organic gum systems which are subject to decomposition at HPHT.
With a view to providing a silicate-based drilling fluid exhibiting good lubricity in oil- and gas-drilling applications, there is provided a silicate drilling fluid comprising 50 to 94.9 volume percent water; 5 to 50 volume percent liquid sodium silicate and 0.1 to 15 volume percent of liquid tetra alkyl ammonium compounds. The drilling fluid may also include high molecular weight polymers for improving rheological properties or decreasing fluid loss. Hectorite clay may be added as a viscosifying agent and other useful materials may be added to the water-based silicate drilling fluid according to the invention.
As is conventional in the art, herein xe2x80x9cliquid sodium silicatexe2x80x9d refers to aqueous silicate solutions prepared at the usual commercial concentrations ranging from about 37 weight percent to 45 weight percent of silicate in water. Likewise, xe2x80x9cliquid tetra-alkyl ammonium hydroxidexe2x80x9d refers to commercially available solutions in which the ammonium compound is present at about 25% by weight in the aqueous solution.
The preferred tetra alkyl ammonium compound is ammonium hydroxide tetra substituted with lower alkyl (methyl to butyl) groups. The currently preferred compound is tetramethylammonium hydroxide (TMAH).
Silicates useful in carrying out the present invention include materials in solution as well as hydrated solids and anhydrous silicates exhibiting molar ratios of SiO2:Na2O (and/or K2O) in the range of 1.5 to 4.0. A particular silicate which has been found useful in silicate-based drilling fluids according to the present invention is D Sodium Silicate (trademark) manufactured by the PQ Corporation (Valley Forge, Penn.). D(copyright) Sodium Silicate exhibits a weight ratio of SiO2:Na2O of about 2.0, and a silicate concentration of 45 percent by weight in aqueous solution.
It is preferred to prepare silicate-based drilling fluids according to the present invention by adding the TMAH lubricant to the silicate mud, i.e., to the pre-mixed water/silicate. In that way, the higher levels of lubricant in the silicate drilling fluid can be achieved, compared with the procedure of pre-mixing the lubricant with silicate before adding the water. TMAH and other tetra alkyl ammonium compound lubricants of the kind comprehended in the present invention are not as soluble in liquid silicate as in a water-diluted silicate drilling fluid. Nevertheless, we have discovered that the lubricant is effective at low concentrations.
With a view to formulating a silicate mud having a stable rheology at high pressure and high temperature, there is added to a silicate-based drilling fluid continuing the tetra alkyl ammonium compound lubricants, a suitable amount of a hectorite clay. To provide HPHT rheology, we have found that hectorite clay in the amount of 1-10 percent ppv should be included in the final formulation.